Fragments of Moloney murine sarcoma virus (MSV) proviral DNA could be activated to transform cells upon addition of viral long terminal report (LTR) DNA's. The cellular homologue (c-mos) of MSV transforming sequences was not active until LTR's were joined to it in a recombinant DNA clone. MSV-induced tumors in Japanese quail, and sera from regressor birds were examined for antibodies. Viral gene products were altered in various tumor lines. Reversion processes were examined in MSV transformed nonproducing cells.